In a process of manufacturing a semiconductor device, photolithography is an important step. During photolithography, a foreign substance deposited on a photo mask precludes a pattern from being accurately transferred. Thus, a pellicle is applied to the photo mask to protect a pattern surface of the photo mask. The pellicle lies at a given distance from the pattern surface of the photo mask. Hence, even if a foreign substance is attached to a surface of the pellicle, an image of the foreign substance is prevented from being transferred during exposure.
However, as the semiconductor device size is reduced, distortion of the photo mask after the application of the pellicle poses a problem. For example, the distorted pellicle may distort the photo mask. Furthermore, the photo mask itself may be distorted. Moreover, the photo mask may be distorted after the photo mask is fixed to a photo mask chuck of an exposure apparatus.
When exposure is carried out using the thus distorted photo mask, accurate transfer of the patterns is difficult. Therefore, there has been a demand for a method for enabling accurate pattern transfer.